Machines for the manufacture of asphalt paving compositions are well-known. Hot-mix asphalt plants, for example, include a means for heating and drying aggregate and a means for mixing the heated and dried aggregate together with asphalt cement or liquid asphalt to form a paving composition.
Hot-mix asphalt plants typically include a rotating aggregate dryer in which aggregate is heated and dried before being mixed with asphalt cement, an asphalt cement supply system and a mixing tower or chamber where the aggregate and asphalt cement are mixed together. The dryer of this apparatus is typically oriented so as to have an upper end and a lower end, so that aggregate which is introduced into the dryer at the upper end will move towards the lower end by gravity flow and by the action of a series of flights mounted on the interior surface of the rotating dryer. A burner is located at the lower end of the dryer, and the hot air and exhaust gases from the burner move toward the upper end of the dryer, against the flow of the aggregate. As the aggregate is tumbled through the exhaust gases, the material is heated and dried. The hot, dry aggregate is discharged from the dryer at the lower end and introduced to a mixing tower or chamber where it is combined with asphalt cement or another binding material to form an asphalt paving composition or asphalt concrete mixture.
As the aggregate is tumbled and dried in the dryer, a quantity of dust is typically created and carried upwardly by the hot gases of combustion. Because of particulate emission regulations, it is unacceptable to exhaust the dust to the atmosphere. Furthermore, depending on the speed of rotation and the temperature at which the dryer is operated, the quantity of dust may represent a significant percentage of the fine aggregate material needed in the particular mix. Therefore, dust collection or recovery systems such as baghouses and cyclone separators are known for removal of the dust from the gas stream before further processing of the combustion gases and/or exhaustion to the atmosphere. The dust which is recovered in the dust recovery system may then be introduced to the mixing tower or chamber for inclusion in the asphalt mix. This introduction of recovered dust has generally been made through a single inlet which deposits the dust at a single point in the mixing chamber. While this single inlet method of dust introduction accomplishes the primary task of return of the dust to the mixing chamber, we have learned that the introduction of dust at more than one point in the mixing chamber allows the dust to be more readily combined with, and more evenly distributed throughout, the asphalt mix than if the dust is introduced at a single point in the mixing chamber.
Accordingly, there is a need to provide an improved apparatus for manufacturing asphalt paving compositions which may avoid or minimize the mixing problems that result from the introduction of recovered dust at a single point in the mixing chamber.